Wide-angle lens system

Abstract

A wide-angle lens system includes a negative first lens group, and a positive second lens group, in this order from the object. The groups are positioned so that the distance between the negative first lens group and the positive second lens group is set as the maximum distance in the wide-angle lens system. The wide-angle lens system satisfies the following conditions: 2.0< fB/f <2.5  (1) 4.5< f 1 /f <6.2  (2) 1.8< f 2 /f <2.0  (3) 1.0< d/f <1.5  (4) wherein fB: the back focal distance of the wide-angle lens system; f: the entire focal length of the wide-angle lens; f 1: the focal length of the negative first lens group; f 2: the focal length of the positive second lens group; and d: the axial distance between the negative first lens group and the positive second lens group.

1. A wide-angle lens system comprising a negative first lens group, and a positive second lens group, in this order from an object,
 wherein said negative first lens group and said positive second lens group are positioned so that said distance between said negative first lens group and said positive second lens group is set as the maximum distance in said wide-angle lens system; and  
 wherein said wide-angle lens system satisfies the following conditions: 
   2.0 <fB/f <2.5  
   4.5 <|f   1   /f |<6.2  
   1.8 <f   2   /f <2.0  
   1.0 <d/f <1.5  
 
 wherein  
 fB designates the back focal distance of said wide-angle lens system;  
 f designates the entire focal length of said wide-angle lens system;  
 f 1  designates the focal length of said negative first lens group;  
 f 2  designates the focal length of said positive second lens group; and  
 d designates the axial distance between said negative first lens group and said positive second lens group.  
 
   
   
     2. The wide-angle lens system according to  claim 1 , wherein said negative first lens group is arranged to be provided with three negative lens elements; and
 wherein said three lens elements satisfy the following conditions: 
   |φ n−3 |<|φ n−1 <|φ n−2 | 
   ν n−3 <ν n−1 <ν n−2    
   31.0<(ν n−1 +ν n−3 )/2<38.0  
 
 wherein  
 φ n−1 , φ n−2 , and φ n−3  designate optical powers of said three negative lens elements in this order from said object; and  
 ν n−1 , ν n−2 , and ν n−3  designate the Abbe numbers of said three negative lens elements in this order from said object.  
 
   
   
     3. The wide-angle lens system according to  claim 1 , satisfying the following conditions:
   0.7 <f   1   /r   12 <1.2  
   0.7 <f   1   /r   21 <1.2  
   |( r   12   −r   21 )/( r   12   +r   21 )|<0.1  
 wherein  
 r 12  designates the radius of curvature of the most image-side surface of said negative first lens group; and  
 r 21  designates the radius of curvature of the most object-side surface of said positive second lens group.  
 
   
   
     4. The wide-angle lens system according to  claim 1 , wherein said negative first lens group comprises a negative meniscus lens element having the convex surface facing toward the object, a biconvex positive lens element, a negative meniscus lens element having the convex surface facing toward the object, a biconcave negative lens element, and a biconvex positive lens element, in this order from said object. 
   
   
     5. The wide-angle lens system according to  claim 1 , wherein said positive second lens group comprises a cemented lens element having a negative lens element and a positive lens element, and a positive lens element, in this order from said object. 
   
   
     6. The wide-angle lens system according to  claim 1 , wherein a diaphragm is provided behind said positive second lens group. 
   
   
     7. The wide-angle lens system according to  claim 1 , wherein a diaphragm is provided between said negative first lens group and said positive second lens group.